The invention relates to a method for determining of absolute plasma parameters of unsymmetrical radio frequency (RF) low-pressure plasmas. The invention further relates to an apparatus particularly for measuring of the radio frequency discharge current at a portion of a plasma reactor acting as ground electrode or earth electrode.
In physics, the meaning of the term "plasma" is that of an ionized gas. In this process, a gas molecule is ionized, i. e. a gas molecule is split up into a free electron and a positive ion (ionized molecule). This process often is an electron collision ionization. The feeding of a radio frequency alternating electric field leads to an accumulation of energy with respect to the electrons for an inelastic collision. The electrons impinge on other gas molecules which are again split up as mentioned before into free electrons and positive ions. A plasma state is formed in the entire reactor by this process. Electrons and ions recombine on the wall since the charge carrier, namely the free electrons and the positive ions, discharge to the walls of the reactor. Thus, a gas molecule is again formed so that an equilibrium is finally established in the reactor.
The radio frequency alternating electric field is generated in the plasma reactor by means of parallel-plates wherein an excited or driven radio frequency (RF) electrode is positioned opposite to an earth electrode. The RF electrode is also called "hot electrode". In the following, the real electrode which electrically forms the earth or ground, which is directly opposed to the RF electrode and which also includes portions of the reactor wall acting as earth, is called "earth electrode". The electrodes of the plasma reactor are normally arranged in a horizontal manner. The RF electrode extends close and in parallel to the bottom wall or to the top wall of the reactor.
In the following, a plasma having an excitation frequency between 10 and 100 MHz and a pressure of 0.1 to 100 Pa, preferably 0.1 to 20 Pa, is called RF low-pressure plasma. Therefore, the gas is held under a predetermined constant pressure (vacuum) in the reactor. It is also possible to continuously feed the gas into the reactor and to continuously discharge the gas from the reactor.
Thus, the plasma processes take place in an evacuated reactor. RF low-pressure plasmas are used in the field of fundamental research and above all in the field of the semiconductor technology. Methods such as plasma etching (PE), reactive ion etching (RIE) and plasma enhanced chemical vapor deposition (PECVD) are for example known.